Thick smoke blankets Sydney as hazard reduction burns conducted

Air quality has reached "hazardous" levels in some parts of Sydney as firefighters conduct controlled burns across the city.

Dozens of hazard reduction burns are being carried out across Sydney and greater NSW by the NSW Rural Fire Service today ahead of the bushfire season.

A large burn north of Kenthurst has sent thick smoke toward the city which has settled over the CBD.

Wind across Sydney has been sending the smoke from the west into the centre of the city. 

NSW Health has warned at risk groups to take extra caution due to the poor air quality in some parts of the basin. 

Chullora and Earlwood in the city's west have recorded the worst air pollution levels.

Those with lung disease, older residents and young children have been advised to avoid outdoor activities.

"Throughout the morning winds have been predominantly coming from the west," Weatherzone meteorologist Tom Hough told 9news.com.au.

"Into the afternoon that will turn more northerly, north-easterly." 

The wind change could see some of the smoke cleared out later this evening.

Aerial video shows the city's famous skyline hidden behind the thick clouds of smoke.

Firefighters have started another burn at Warringah which is expected to send more smoke into the basin.

Sydneysiders can keep track of planned burns on the RFS website.